From our limited human perspective, it seems impossible that God would become a man, die on a cross, rise three days later, then ascend into heaven. It seems even more impossible that our salvation—our only chance to spend eternity in heaven—is rooted in our belief in Jesus Christ who died on that cross for our sins. Humanly speaking, how could the death of one man so long ago determine the destiny of every human being ever created? God's act of redemption makes no sense to the natural mind. Human wisdom cannot comprehend the cross.
Human wisdom may show us the problems of life, but it fails to give us the solutions. The root of all our problems is sin, which humanity refuses to acknowledge. The unrepentant heart justifies and explains away sinful behavior.
People consider the cross foolishness because admitting their sinfulness and their need for Christ's redemption means they must also surrender to God. They must give up their own wisdom and glory for God's. And most people are unwilling to admit God's ways are better than theirs.